A re-examination of stratigraphy and age of the Lower Pleistocene Bushi Formation in the Kaji Hills, western Kanto Plain, central Japan

Abstract

We investigated the stratigraphy of the Lower Pleistocene Bushi Formation in the Kaji Hills, western Kanto Plain, central Japan. A total of 27 tephra layers, including some widespread tephras, are distinguished. The Bushi Formation is divided into five lithological units (A to E) on the basis of the lithofacies characteristics of depositional cycles in each unit. A total of eight marine beds (M1 to M8) are recognized from lithofacies and diatom assemblages. The depositional age of each unit is estimated by correlation of widespread tephras and marine beds, as follows: Unit A: 2.5-2.4 Ma, Unit B: 2.4-2.2 Ma, Unit C: ~1.8 Ma, Unit D: 1.7-1.6 Ma, and Unit E: 1.6-1.4 Ma. Alternating layers of shallow-marine and fluvial deposits in the interval between the Azu-Koen4 tephra layer and the M5 marine bed can be considered as a depositional cycle that represents eustatic sea-level change during marine oxygen isotope stages 93 to 89.